1. Field of the Invention
The present invention relates to a discharge lamp, and more specifically to a discharge lamp having a discharge space bounded by an external tube and an internal tube.
2. Discussion of the Background
An excimer reflector lamp is described in DE-A 196 13 502. The lamp has a closed discharge space constructed as an annular gap between two quartz glass tubes arranged coaxially in relation to each other. The discharge space contains a filler gas that forms excimers under discharge conditions. An external electrode in the form of a net is provided on the external wall of the external quartz glass tube. An internal electrode is formed by a spiral wire lying in the internal wall of the internal quartz glass tube. Upon applying a high voltage between the internal and external electrodes, so-called excimers are formed in the filler gas of the discharge space. Due to the chemical composition of the excimers, the excimers emit essentially monochromatic UV radiation and do not emit coherent radiation. The discharge between the external and internal electrodes passes through the discharge space and forms fine threads of current, which are called filaments.
In conventional excimer reflector lamps, the internal electrodes can be installed in the form of wire spirals. However, the filaments are concentrated in the area of the wire spirals. For applications in which a homogenous distribution of the filaments is desired, it may also be necessary to require uniform contact of the wire spirals with the internal walling of the internal tube.
Another excimer radiation lamp is shown in EP-A1 767 484, which discloses an excimer radiation lamp having an internal electrode constructed in the form of a metal tube with a longitudinal slot running in the direction of the radiation axis. For installation of the internal electrode, the metal tube is rolled together and inserted into the internal tube. In this manner, the internal electrode is caused to firmly contact the inner wall of the internal tube. As a result the, the filaments formed in the discharge space are generally distributed homogeneously. However, the filaments tend to wander along the longitudinal slot with a vertically oriented radiation lamp axis.
Accordingly, one object of the present invention is to provide a novel discharge lamp having an internal electrode that is easy to install.
It is another object of the present invention to provide a novel discharge lamp that lies fast on an internal wall of the discharge lamp.
It is yet another object of the present invention to provide a novel discharge lamp in which the filaments are homogeneously distributed in the discharge space, even when the discharge lamp is vertically mounted.
These and other objects are achieved according to the present invention by providing a discharge lamp that includes an external tube formed of dielectric material and an internal tube formed of dielectric material. The external tube and the internal tube define a discharge space. An external electrode is arranged proximate the external tube, and a helical internal electrode is arranged adjacent to an inner wall of the internal tube. The internal electrode includes an elastic spiral band, which in a released state, has an external diameter, D, that is larger than an internal diameter, I, of the internal tube.
The helical or spiral shape of the internal electrode advantageously prevents the filaments from wandering independently of the orientation of the discharge lamp. Additionally, since the internal electrode is in the shape of a spiral band, the internal electrode achieves a flat contact on the internal wall of the internal tube, the electric field in the discharge space is homogenized, and the distribution of the filaments is homogenized.
Moreover, the elastic deformability of the spiral band facilitates installation in the internal tube since the spiral band can be extended for placement in the internal tube, rolled together more compactly, and subsequently released so that it expands outward and pushes against the inner wall. This is possible because the spiral band in the released state has an external diameter that is greater than the diameter of the internal tube. As used herein, the term xe2x80x9cexternal diameterxe2x80x9d means the maximum diameter of the cross section perpendicular to the long axis of the spiral. The spiral band is in the released state if no external forces are acting upon it (i.e., the spiral band is not compressed).
In addition, the internal electrode serves as a reflector, and it is suited for both straight and curved internal tubes.
Advantageously, the spiral band has a slot or gap between adjacent winding segments. The width of the gap (i.e., the gap width) is preferably between 0.2 mm and 5 mm. As the width of the slot decreases, the distribution of the filaments in the discharge space becomes more homogeneous and the reflecting action of the spiral band becomes more effective. However, as the width of the slot narrows, the deformability of the spiral band diminishes, which makes installation of the spiral band more difficult. Preferably, the width of the slot is between 0.5 mm and 2 mm.
In a preferred embodiment, the spiral band has a flat cross section and a breadth of between 3 mm and 30 mm. More preferably, the breadth of the spiral band is between 5 mm and 10 mm. Increasing the breadth of the spiral band increases and enhances the field homogenizing and reflecting function of the internal electrode. However, increasing the breadth of the spiral band decreases the deformability of the spiral band.
Preferably, the thickness of the spiral band is between 0.1 mm and 1 mm. The spiral band is preferably made of high grade steel or of a spring steel.
In a preferred embodiment, the outer side of the spiral band (i.e., the side of the spiral band that faces the discharge space) is coated with a material that forms a surface that reflects UV radiation. The reflectivity of the surface can be adjusted by changing the type of material or by adjusting the degree to which the surface is polished.
The internal electrode formed by the elastic spiral band is especially suited to a discharge lamp with a bent internal tube. Given its pliability, the internal electrode can adapt to the bends in the internal tube. The bends in the tube can be either a kink or a continuous curve. For example, the curves can be circular, semicircular, banana-shaped or U-shaped.
The spiral band of the present invention is particularly well suited for use in a discharge lamp having a discharge space filled with a gas that forms excimers when the electrodes of the lamp discharge.